The present invention relates to a colour image projection apparatus comprising illumination means, image display means comprising a plurality of image display panels each illuminated with a different colour light and each having an array of pixels arranged in rows and columns and operable to produce a display output, means for combining the display outputs from the plurality of display panels, a projection lens for projecting the combined outputs on to a projection screen to produce an image, and electro-optical deflection means which is switchable between two modes in which for each panel the light rays from a row of pixels of the display panel and passing through the deflection means are shifted relative to one another so as to displace the projected image of the row of pixels on the projection screen, and which electro-optical deflection means comprises a birefringent element which follows the combining means and segmented polarisation switch means which precedes the birefringent element.
Such apparatus may be used for large area display of a broadcast video programme, or an image signal from a storage medium, for example an optical disc or a computer medium. The image display panels used in such apparatus may be matrix LCD panels consisting of a layer of liquid crystal material sandwiched between two substrates and preceded by a polarizer and succeeded by an analyser. The panels comprise a large number of pixels, i.e. individual picture elements, arranged in rows and columns and addressed by a system of row and column electrodes carried on one or both of the substrates through which drive signals are supplied to the pixels. The matrix LCD panel may be of the active type in which each individual pixel is provided with a switching device, typically a thin film transistor (TFT) or a thin film diode (TFD). For a full colour projection display then the apparatus, as with known LC projection apparatus, may have three image display panels each of which is illuminated with light of a respective primary colour, red, green and blue, which may be obtained by splitting the output from a white light source into its primary colour components, with each panel modulating its respective colour light component according to supplied display information, e.g. a video signal, and the modulated outputs being combined and projected onto the screen to produce a full colour image.
In conventional projection apparatus, the number of pixel rows in the image display panels should correspond to the number of image lines in the supplied video signal to ensure that all image information is utilised in producing the viewed display. In a standard TV signal each frame consists of two interlaced fields transmitted sequentially, each comprising half the total number of display lines. The information for all the odd numbered lines is transmitted and then the information for all the even numbered lines. In a matrix LC display panel the pixel rows each display the information of a video line and all pixel rows, odd and even, are continuously illuminated so light passes through the pixels in the even rows during the time when only odd rows information is available, and vice versa. Because each pixel of the panel passes light at each moment, old image information is interwoven between the most recent image information which leads to display artefacts in a picture with fast moving parts. Various ways have been devised to mitigate the artefacts. For example, during each field period when new information is provided for only half of the pixel rows, the adjacent rows may be provided with the same information. However, this results in a decrease of picture contrast and resolution.
A liquid crystal video projector is disclosed in the paper entitled "A practical wobbling device using Ferro-Electric Liquid Crystals for realising high resolution projectors"by A. Yasuda et al in: "Asia Display 1995", pages 79-82, wherein the odd field of a picture frame is displayed by the same pixel rows as the even field. To that end the projector includes a so-called wobbling device serving as an electro-optic deflection means. This device consists of a segmented polarisation switch and a birefringent plate. The polarisation switch, comprising a ferro-electric liquid crystal panel, is drivable to rotate the plane of polarisation of the light from the display panel through ninety degrees and the birefringent plate changes the propagating direction of the light rays depending on the orientation of the plane of polarisation. The segments of the polarisation switch are operated in sequence such that the plane of polarisation of light from the display panel passing through each segment, corresponding to a group of pixels rows, is switched in turn. The driving of the wobbling device is synchronised with an interlaced image signal, which contains two fields for each picture frame and, for example 25 or 30 frames per second, supplied to the display panel. The picture lines in the projected display corresponding to the second field are shifted to the areas between the pixel rows, producing interlaced images, and the number of displayed horizontal lines appears to be doubled. In this way, to display a standard TV-signal, NTSC or PAL, a display panel can be used that has a number of pixel rows half that of a conventional display. On the other hand a High Definition picture, with for example 1148 horizontal lines, can be displayed by a panel having only 574 pixel rows and originally intended for displaying a standard TV signal. With such a projection apparatus even numbered lines are not displayed when only the odd-numbered line information is available, and vice versa. In a colour projection arrangement described in this paper, the electro-optical deflection means, comprising the polarisation switch and the birefringent plate, is arranged between the optical combining means, in the form of a dichroic prism, for combining the outputs from the three LC panels into a single beam and the projection lens.